1. Field of the Invention
The present invention relates to an optical reticle substrate inspection apparatus that inspects a defect and a line width of a reticle substrate and a beam scanning method thereof, particularly, to an optical reticle substrate inspection apparatus for improving stability when a laser light source having a short wavelength is used and a beam scanning method thereof.
2. Description of the Related Art
In the optical reticle substrate inspection apparatus that inspects a defect and a line width of a reticle substrate, there exists an apparatus in which detection light is converged onto a substrate to detect the defect and the line width by transmission light or reflection light. In such a apparatus, a spot of a laser beam (a beam spot) is scanned in one direction parallel with a surface of the reticle substrate (hereinafter, this direction is referred to as a Y-axis direction), and a stage on which the reticle substrate is mounted is moved in an isokinetic manner in a direction parallel with the surface of the reticle substrate and a perpendicular direction to the Y-axis direction (hereinafter, this direction is referred to as an X-axis direction), and thus inspecting the defect and the like of the reticle substrate. As a scanning method of the beam spot, a method in which the beam spot is optically scanned is generally used in order to process inspection in a short time, and the beam scanning method using an acoustooptical element has been conventionally adopted.
In such a beam scanning method, aberration occurs to an angle of the scanning beam when the beam is converged by a normal cylindrical lens, and defocus occurs on the reticle substrate surface. For this reason, a method is adopted where the beam is converged using a lens effect by the acoustooptical element to reduce the defocus. A method converging the beam using the two acoustooptical elements is described in The U.S. Pat. No. 3,851,951, for example. And a substrate inspection apparatus that adopted the method is described in Japanese Patent Laid-Open (unexamined) No. Hei 6-294750. FIG. 1 is a schematic view showing the conventional optical reticle substrate inspection apparatus.
In the optical reticle substrate inspection apparatus, a laser light source 1, an acoustooptical element 2 and a group of cylindrical lenses 3 are arranged in this order in a rectilinear direction of a laser beam output from the laser light source 1. The acoustooptical element 2 is arranged so as to scan the laser beam output from the laser light source 1 in a perpendicular direction to the straight line by frequency modulation, and the group of cylindrical lenses 3 is arranged so as to magnify the laser beam output from the acoustooptical element 2 only in the scanning direction by the acoustooptical element 2.
An acoustooptical element 4 which gives a cylindrical lens effect to the laser beam output from the group of cylindrical lenses 3 is further provided to the optical reticle substrate inspection apparatus. A transducer 19 which oscillates ultrasonic to the acoustooptical element 4 is attached to one end portion of the acoustooptical element 4. The acoustooptical element 4 is arranged such that a side where the transducer 19 is attached, that is, a side where the ultrasonic is input, is closer to the group of cylindrical lenses 3. Furthermore, a group of the cylindrical convex lenses 18 that converges the laser beam output from the acoustooptical element 4 is arranged in the optical reticle substrate inspection apparatus. Moreover, a relay lens 24 which propagates the laser beam passed through the convergence spot to an optical system (not shown) in a post-step is arranged on a position apart from the convergence spot formed by the group of cylindrical convex lenses 18. In addition, an objective lens (not shown) is arranged between the optical system and the reticle substrate being an object to be inspected, and a detector (not shown) that detects intensity and the like of the laser beam passed through the reticle substrate is further provided.
In the optical reticle substrate inspection apparatus constituted in this manner, scanning is performed in an angle made by an incident direction and an output direction to the laser beam output from the laser light source 1 by utilizing the frequency modulation by the first acoustooptical element 2. An optical path of the laser beam transits from a path 12 to a path 13 due to this process. When the laser beam has its optical path on the path 12, the laser beam is magnified by the group of cylindrical lenses 3 and output as a laser beam 30 with a width in the scanning direction.
Further, transducer 19 outputs a series of ultrasonic which is swept such that a wavelength lengthens in linear state as the passage of time, that is, the wavelength in a forefront portion 21 becomes shorter than that of an aftermost portion 20. A plurality of parallel lines between an aftermost portion 20 and a forefront portion 21 in FIG. 1 show that the wider the distance between the lines the longer the wavelength. The second acoustooptical element 4, when the laser beam 30 is made incident thereto, outputs the laser beam 30 while converging it with functioning as a cylindrical convex lens by the ultrasonic oscillated from the transducer 19. The laser beam 30 output from the acoustooptical element 4 is further converged by the group of cylindrical convex lenses 18 to form a convergence spot 22. When the laser beam has its optical path on the path 13, a convergence spot 23 is formed on a position off from the convergence spot 22 in the scanning direction. The laser beam 30 passed through the convergence spot 22 is made incident to the relay lens 24 while magnifying its width again. Then, the convergence spot 22 is imaged on the reticle substrate via the optical system and the objective lens, and the detector detects the intensity and the like of the transmission light.
Note that the same detection can be performed even if the acustooptical element 4 is arranged such that the side where the ultrasonic is input is made far from the group of cylindrical lenses 3 and the ultrasonic is swept such that the wavelength becomes shorter in linear state as the passage of time.
However, in recent years, although higher resolving power has been demanded for an apparatus for inspecting the reticle substrate with demand for finer pattern, there is a problem that the conventional optical reticle substrate inspection apparatus cannot provide sufficient high resolving power.
As a method of obtaining the high resolving power, a method is considered where the convergence spot size on the reticle substrate surface is made small by shortening the wavelength of the detection light. Progress of shorter wavelength has been made also in the field of exposure due to advancement in technology. It is important that the wavelength of the detection light is shortened to make it close to that of an exposure light because the way how the defect appears changes depending on a detection wavelength. However, in the case of shortening the wavelength of the detection light, a usable material of the acoustooptical element is limited due to a problem of material absorption regarding the scanning method where convergence is performed by the conventional acoustooptical element. For example, since tellurium dioxide, which has been generally used as the acoustooptical element, does not transmit the detection light having the wavelength of 300 nm or less, the acoustooptical element made of quartz, which has a sonic speed of 5960 m/sec being about ten times faster than comparing to tellurium dioxide, needs to be used. However, there is a problem that a focal length lengthens when the acoustooptical element made of quartz is used, because a focal length of a cylindrical lens effect by the acoustooptical element is inversely proportional to the wavelength of light output from the light source and proportional to a square of the sonic speed.
It is an object of the present invention to provide the optical reticle substrate inspection apparatus that can improve scanning stability when the laser light source having a short wavelength is used and the beam scanning method thereof.
An optical reticle substrate inspection apparatus according to the present invention comprises a laser, a first acoustooptical element which scans a laser beam output from said laser, a second acoustooptical element which generates a virtual image with a concave lens effect to said laser beam output from said first acoustooptical element, a concave lens arranged on the output side of said laser beam of said second acoustooptical element, and an optical system which images said virtual image on a reticle substrate being an object to be inspected. The concave lens magnifies said laser beam in a perpendicular direction to the scanning direction by said first acoustooptical element.
In the present invention, the first acoustooptical element performs laser beam scanning. Further, the second acoustooptical element generates a virtual image whose aberration is reduced to the laser beam, and the concave lens adjusts the shape of the virtual image in a circle, for example. Then, the optical system images the virtual image on the reticle substrate. As described, in the present invention, since not the convergence spot but the virtual image is imaged on the reticle substrate and its scanning is performed, stable scanning can be performed even if the laser beam having a short wavelength is used.
A beam scanning method of an optical reticle substrate inspection apparatus according to the present invention comprises the steps of generating a virtual image with an acoustooptical element to a laser beam, and imaging said virtual image on a reticle substrate being an object to be inspected.